It’s official: The five-second rule is down for the count

In most thorough debunking yet, the principle but not the rule stands test of time.

Who hasn’t tried to salvage a
mouthwatering morsel tragically lost to a germ-covered floor by blurting
“FIVE-SECOND RULE!” before diving to the rescue? Even the most
intelligent among us might be apt to cite the tenet with the fervor of
an emergency responder. But let’s face it: the chances that some magical
cut-off of five seconds will spare food from irreparable defiling was
always far-fetched. Still, if you need hard-hitting evidence,
researchers have now provided a comprehensive study that proves the rule
is down for the count.

After analyzing several floor materials, food
types, contamination methods, and resting times, researchers at Rutgers
show conclusively that food can easily become contaminated in less than
five seconds. In fact, contamination may take less than one second in
some scenarios. In a small consolation, the researchers note that the
general theory behind the debunked rule—that longer resting time on a
contaminated surface will increase contamination—stands.

“Our data clearly showed that contact time
does influence bacterial transfer, with more bacteria transferred at
longer times,” food scientists Robyn Miranda and Donald Schaffner report
in the journal Applied and Environmental Microbiology.
However, they conclude “that other factors including the nature of the
food and the surface are of equal or greater importance.” And in the end
“some transfer takes place ‘instantaneously’ at times <1 s,
disproving the ‘five second rule.’”

The study isn’t the first to quash the popular rule. A few other small studies and experiments—one published, one not, and one on the TV show Mythbusters—have
proven that certain foods dropped on particular surfaces can become
contaminated in fewer than five seconds. But the authors of the current
study wanted to crush the rule “in an extensive and comprehensive
manner” and completely tease out the factors of cross-contamination.

So the researchers dropped chunks of
watermelon, bread, buttered bread, and gummy candy from 12.5 cm onto
contaminated surfaces made of either stainless steel, wood, ceramic
tiling, or carpet. Each faux floor was coated in one of two dried
culture buffers containing Enterobacter aerogenes B199A bacteria. Even though it’s harmless, Enterobacter aerogenes mimics the attachment skills of Salmonella,
a common food pathogen. And food was left for either less than one
second, five seconds, 30, or 300. Each condition was repeated 20 times,
resulting in a total of 2,560 data points.

Each variable altered the transfer rate of
bacteria to the plopped food. Generally, tile and stainless steel had
the highest transfer rates. Wood’s transfer rate varied with foods,
while carpet was the least likely to give up bacteria to any food that
landed.

Watermelon chunks, a moist food with smooth
surfaces, was the best at soaking up contamination from the floor. It
became contaminated instantly in all but one condition—on carpet
contaminated with bacteria in a peptone buffer. Gummy candy was the best
at resisting contamination. But, the authors note, that may change if
the candy is dropped from greater heights, thereby increasing the
pressure and splat-factor.

Overall, there were a few instances when foods
didn’t become contaminated within five seconds—notably the gummy bears
on carpet. However, the authors conclude this exception isn't enough to
prove the rule. They write:

The risk of illness resulting from
deciding to consume food that has fallen on the floor will depend on
factors including prevalence, concentration and type of organism, the
nature of the food (especially moisture), the nature of the surface
topology as well as the length of time the food is in contact with the
surface... The 5-second rule is a significant oversimplification of what
actually happens when bacteria transfer from a surface to food.